A leuco color forming type heat-sensitive recording material is generally used as a recording material for heat-sensitive recording systems. However, such a heat-sensitive recording material has defects in that color is formed at unexpected areas due to rough handling, heating or attachment of solvents and the like, and accordingly recorded images are often stained. Recently, a diazo color forming type heat-sensitive recording material, which does not have the above defects, has been studied very extensively. It is disclosed, for example, in Japanese Patent Application (OPI) No. 123086/82 (the term "OPI" as used herein means an "unexamined published application") and in the bulletin of The Journal of Image Electronic Society, 11, 290 (1982) that heat-recording is conducted on a recording material containing diazo compounds, a coupling component and a basic component (including substances which become basic by heating) and thereafter the recording material is exposed to light to decompose the unreacted diazo compounds, resulting in cessation of color formation. In fact, color formation at the non-recorded area can be stopped (hereinafter referred to as "fixation") by this method. However, in the recording material of this type, pre-coupling gradually occurs during storage before use, and therefore unfavorable color formation (fog) occurs in many cases. In this case, the precoupling can be prevented by making one of the color forming components exist in the form of discontinuous particles so as to prevent it from contact with other components. However, the storage stability before recording is not sufficient and there is a defect that the color forming property by heating decreases. Further, it is disclosed, for example, in Japanese Patent Application (OPI) No. 123086/82 that diazo compounds and a coupling component are incorporated into each layer separately in order to minimize the contact between the color forming components. Storage stability before recording can be improved by the above method, but the color forming property by heating largely decreases and the recording material of this type can not respond to high speed recording having a short pulse width and thus is not suitable for practical use. Still further, in order to realize the satisfactory storage stability before recording and satisfactory color forming property by heating, it is known that any one of a coupling component and a basic substance can be encapsulated by a non-polar wax-like substance (as disclosed in Japanese Patent Application (OPI) Nos. 44141/82 and 142636/82) or by a hydrophobic high molecular weight substance (as disclosed in Japanese Patent Application (OPI) No. 192944/82) in order to separate the coupling component and/or basic substance from other components. This encapsulation comprises dissolving a wax or a high molecular weight substance in suitable solvents, and dissolving or dispersing a color forming component into the thus obtained solution to form capsules, whose function is different from that of capsules where a core is covered with a shell. For the above reason, when a color forming component is dissolved to form capsules, the color forming component does not become a core substance for a capsule, but does disperse with the encapsulated substance homogeneously. Therefore, pre-coupling occurs at the wall of the capsule during storage, resulting in poor storage stability before recording. As a result, when a color forming component is dispersed to form a capsule, it is necessary to heat and melt the wall of the capsule in order to cause the color forming reaction. Thus results in poor color forming properties by heating. Further, the above-described method has a manufacturing problem in that the solvents used for dissolving the wax or the high molecular weight substance should be removed after the capsule is formed. Therefore the above method is not sufficiently satisfactory.
In order to solve the above problems, another encapsulation method has been studied, and it was found that the problem could be solved by an excellent heat-sensitive recording material prepared by incorporating at least one of the color forming components into a core substance and forming a wall around the core substance by polymerization to obtain microcapsules.
However, in the heat-sensitive recording material prepared by the above-described microencapsulation method, the optical density at the area of recorded images appears to decrease upon long storage after heat-recording in many cases. Therefore, further improvement has been required.